Heat flux extracted from steel plate with iron oxide scale by water boiling

A new method has been developed to measure heat flux extracted from steel surface with iron oxide scale using the mass change of water and the standard enthalpy of evaporation for water at 100ºC. The apparatus was basically composed of a furnace, an electronic balance and a video camera. Samples used were ultra-low carbon steel plates, which were oxidised in air at 850ºC in the furnace so as to have iron oxide scale (mainly FeO) with desired thicknesses (40 - 103 m), and then were moved onto the electronic balance. About 3 g of water was dropped onto the sample, and the mass change of water was measured by the balance and the temperature change inside the sample measured by a thermocouple. In addition, boiling behaviour of water was recorded by the video camera. The mass and temperature changes well corresponded to the video image. The quench points for the samples with scale 40, 58, 77 and 103 m thick were 131, 167, 121 and 182ºC, respectively, and the respective heat fluxes just before quench were 31, 35, 33 and 43 kWm ^-2 . It seems that there are no clear effects of scale thickness on the quench point and the heat flux just before quench. Assuming a quasi-steady state, Fourier's law has been applied to estimate the thickness of water vapour film during film boiling; as a result, it is found that the thickness decreases from ca. 100 m to ca. 20 m on cooling. These thicknesses are comparable to previously-reported results by direct observation, which suggests that the present estimation is not so unreasonable. Estimation of temperature distribution in the sample suggests that thermal conduction in the water vapour film determines the total heat transfer; thus, there would be no clear effects of scale thickness on the quench point and the heat flux just before quench.